More font paths test coverage.

Specifically, the new cases check the correctness of paths generated by
font_files / font_url when relative_assets is on in build mode, and when
a custom fonts_dir is specified in preview mode.
Merge pull request #886 from pwnall/font_paths

More font paths test coverage.
merge more into core, use gem availability to autoload
Merge pull request #891 from middleman/magic_more

Merge more into core, use gem availability to autoload support